The Prophet ﷺ said, "Neither Fara' nor 'Atira) is permissible)." Al-Fara' was the first offspring (they got of camels or sheep) which they (pagans) used to offer (as a sacrifice) to their idols. 'Atira was (a sheep which used to be slaughtered) during the month of Rajab.  

عَنِ النَّبِيِّ ﷺ قَالَ لاَ فَرَعَ وَلاَ عَتِيرَةَ قَالَ وَالْفَرَعَ أَوَّلُ نِتَاجٍ كَانَ يُنْتَجُ لَهُمْ كَانُوا يَذْبَحُونَهُ لِطَوَاغِيتِهِمْ وَالْعَتِيرَةُ فِي رَجَبٍ